Article pubs.acs.org/JPCC
Photoinduced Electron Transfer between Ruthenium-bipyridyl Complex and Methylviologen in Suspensions of Smectite Clays Teruyuki Nakato,*,† Shoko Watanabe, Yasuhiro Kamijo, and Yoshihiro Nono† Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2−24−16 Naka-cho, Koganei-shi, Tokyo 184−8588, Japan. S Supporting Information *
ABSTRACT: We examined photoinduced electron transfer (PET) in multicomponent aqueous suspensions composed of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)32+, photocatalyst), methylviologen (1,1′-dimethyl-4,4′-bipyridinium dication, MV2+, electron acceptor), and ethylenediamine tetraacetate (EDTA, sacrificial electron donor) together with particles of smectite-type clays although previous studies indicated inhibition of the electron transfer from Ru(bpy)32+ to MV2+ in the presence of clay particles. Clays with different lateral particle sizes were compared: hectorite (Hect) and saponite (Sapo) with small particle sizes (∼30 nm) and fluorohectorite (FH) and montmorillonite (Mont) with large particle sizes (>0.1 μm). Clay particles were flocculated and were settled in many cases after the addition of Ru(bpy)32+, MV2+, and EDTA species, and the Ru(bpy)32+ and MV2+ cations were almost all adsorbed on the clay particles. When Hect and Sapo were used, reduction of MV2+ was observed on the aggregated clay particles upon visible light irradiation indicating the occurrence of PET from Ru(bpy)32+ to MV2+. However, the reaction was not observed for the samples where the clay particles were not settled. When FH and Mont were used, PET was not observed irrespective of the flocculation of clay particles. These results demonstrated that PET from Ru(bpy)32+ to MV2+ in the presence of clay particles is possible when the clay particles with small sizes are appropriately aggregated to allow interparticle electron hopping.
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INTRODUCTION Photoinduced electron transfer (PET) is a key reaction of photoenergy conversion. While numerous donor−acceptor pairs have been studied, the PET reaction from Ru(bpy)32+ to MV2+ is one of the most famous reactions because it is recognized as a model of solar energy conversion.1,2 In this system, Ru(bpy)32+ molecules excited by visible light around 450 nm transfer electrons to MV2+ dications to generate methylviologen radical cations (MV+•), which can reduce H+ cations to H2 molecules. However, the PET reactions of Ru(bpy)32+−MV2+ and many other donor−acceptor pairs do not proceed efficiently in homogeneous solutions because of fast backward electron transfer.3,4 Thus, we scarcely yield the photoproducts continuously under stationary illumination. To overcome the inefficiency, addition of sacrificial reagents to the solution has been investigated. For the pair of Ru(bpy)32+ and MV2+, EDTA and triethanolamine (TEOA) have often been used as the sacrificial electron donors, which enables accumulation of the MV+• ions as shown in Scheme 1 or subsequent continuous reduction of H+ ions to H2 molecules in the presence of an appropriate cocatalyst such as Pt.5−7 As shown in the scheme, the photoexcited triplet state of Ru(bpy)32+ is oxidatively quenched by MV2+ to generate Ru(bpy)33+ and MV+•. Ru(bpy)33+ reforms Ru(bpy)32+ in the dark through the reaction with the sacrificial donor EDTA. © 2012 American Chemical Society
Scheme 1. Schematic Representation of the Visible-LightInduced Electron Transfer in the Ru(bpy)32+/MV2+/EDTA System
Another important key to efficient PET reactions is utilization of heterogeneous reaction media which regulate location of the reactant molecules to stabilize the charge separation between the donor and acceptor.8−17 In fact, numerous heterogeneous systems have been examined for PET between Ru(bpy)32+ and MV2+. Examples are organic polymers,18−31 micelles,32−34 and inorganic porous oxides such as zeolites and porous glasses.35−47 These heterogeneous media realize stabilization of the photogenerated MV +• species.26,35−38,40,41,44 Also, the use of sacrificial donors achieves efficient accumulation of the MV+• species and the reductive Received: December 9, 2011 Revised: March 30, 2012 Published: March 30, 2012 8562
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more fluid than those given by other clays with large particle sizes.77 We have also recently found that MV2+ molecules adsorbed on Laponite particles can effectively accept electrons from phase-separated photoexcited semiconductor particles.87 These findings suggest that suspensions of unmodified clay particles can mediate PET from Ru(bpy)32+ to MV2+ if we appropriately set the particle size of the clay minerals and if we adjust the dispersed/aggregated conditions of the clay particles. We report herein the PET reaction between the Ru(bpy)32+ and MV2+ species all adsorbed on the smectite-type clay particles with different particle sizes in the presence of EDTA as a sacrificial donor in aqueous suspensions. The reaction proceeds only in the samples where the clay particles with small sizes are flocculated. The reactive samples do not show clear evidence for segregation of Ru(bpy)32+ and MV2+, and we ascribe the occurrence of the PET reaction to the aggregated state of clay particles.
generation of H2 from H+ and of H2O2 from O2 in the heterogeneous systems.18,20−23,28−31,41 Smectite-type clay minerals have been recognized as unique inorganic matrixes among the heterogeneous media.13,48,49 They have chemically and physically stable crystalline layered lattices with expandable interlayer regions, and thus they incorporate various guest molecules into their two-dimensional interlayer spaces.50 They form stable aqueous colloids, where negatively charged platy clay particles that can adsorb cationic molecules are dispersed through delamination of the clay crystals.51 Thus, the smectite-type clays provide characteristic microenvironments that are different from those provided by rigid inorganic porous materials such as zeolites. In fact, the interlayer spaces of the clays allow a variety of photochemical reactions in which specific behavior such as selective formation of unusual photoproducts has been discovered.52−60 The colloidal suspensions of the clays mediate unusual photoprocesses such as controlled photoinduced electron/energy transfer among porphyrin molecules.49,61−65 The smectite-type clays provide specific surface and interlayer environments also for the Ru(bpy)32+ or MV2+ molecules so that characteristic photochemical/photophysical behavior that is not observed in homogeneous solutions has been realized.66−81 For the PET reaction between the Ru(bpy)32+ and MV2+ species, however, the smectite-type clays have been recognized as inappropriate heterogeneous media. This has been ascribed to the segregation of the Ru(bpy)32+ and MV2+ molecules. This phenomenon is regarded as demixed adsorption of different molecules with dissimilar molecular structures onto the clays. The segregated species are spatially separated from each other and are hindered from contact through immobilization by different clay platelets; hence, photochemical communication such as electron/energy transfer between the segregated donor and acceptor molecules is suppressed. This has been found by Ghosh and Bard in colloids of hectorite and montmorillonite clay67 and is supported by later works.76,82−85 Detellier and coworkers have reported photoinduced hydrogen generation in colloids of montmorillonite clay by applying the PET reaction between Ru(bpy)32+ and MV2+ in the presence of a sacrificial donor, and they assigned only the external surfaces of the clay particles to the reaction sites on the basis of the segregation confirmed by X-ray diffraction (XRD).83−85 In their studies, because the amount of Ru(bpy)32+ and MV2+ cations added to the clay suspensions exceeds the cation exchange capacity (CEC) of the clay, the PET reaction can be interpreted as the reaction between the adsorbed and dissolved molecules or among only the aqueous species. Because the segregation is demixed clustering of the Ru(bpy)32+ and MV2+ cations on the negatively charged clay surfaces under densely adsorbed conditions, perturbation of the clustering has been attempted through coadsorption of polymers,75,86 and this method enables the PET reactions on the clay surfaces.80,82 Nevertheless, several studies have suggested that the smectite-type clay particles are utilizable as heterogeneous media for the PET reaction of Ru(bpy)32+ and MV2+ species without complicated modifications such as the polymer coadsorption. Turro et al. have reported that colloids of synthetic hectorite (Laponite) whose particle size is much smaller than that of montmorillonite stabilize the photoexcited state of Ru(bpy)32+ through suppression of the interactions between the adsorbed Ru(bpy)32+ molecules.71 This effect is enhanced when the clay particles are flocculated. The Laponite particles have been reported to provide microenvironments
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EXPERIMENTAL METHODS Materials. We used four smectite-type clay minerals having different particle sizes. Synthetic hectorite (Laponite RD, Rockwood Additives Ltd., United Kingdom) and synthetic saponite (Sumecton SA, Kunimine Industries Co., Japan) are characterized by small particle sizes, and synthetic fluorohectorite (NHT, Topy Industries Ltd., Japan) and natural purified montmorillonite (JCSS−3101 reference clay sample of the Clay Science Society of Japan, Tsukinuno mine, Yamagata Prefecture, Japan) have larger particle sizes. Hectorite (Hect) and saponite (Sapo) were used as received. Fluorohectorite (FH) provided as a suspension was purified by the method of Miyamoto et al.88 First, the as-received suspension was centrifuged at 10 000 rpm for 60 min by which the sample was separated into three phases: upper supernatant, intermediate viscous sol, and lower sediment phases. The sol phase was picked up and dialyzed with water and then was dried at 333 K. Montmorillonite (Mont) was treated with a NaCl solution, was washed with water, and was dried at 333 K. Table 1 lists the ideal formulas, the cation exchange capacities Table 1. Clay Minerals Used in the Present Study mineral Hect Sapo FH Mont
commercial name Laponite RD Sumecton SA NHT JCSS−3101
ideal formula
CEC/ meq g−1
mean particle size/μm
Na0.33Mg1.67Li0.33Si4O12H2
0.74
0.03
Na0.33Mg3Si3.67Al0.33O12H2
0.72
0.03
Na0.33Mg2.67Li0.33Si4O10F2 Na0.33Al1.67Mg0.33Si4O12H2
1.2 1.2
2.2 0.2
(CECs), and the particle sizes of the clay minerals used.88−90 Dichloride salts of Ru(bpy)32+ and MV2+ were purchased from Aldrich Co. and Tokyo Kasei Co., respectively, and were used as received. EDTA disodium salt and TEOA were obtained from Wako Pure Chemical Co. and Kanto Chemical Co., respectively, and were used without further purification. A multicomponent suspension composed of Ru(bpy)32+, MV2+, EDTA, and clay was prepared with the following procedure. First, a colloid of exfoliated clay particles was prepared by adding clay powder to water. The clay concentration was set to 5 g L−1. Next, aqueous solutions of Ru(bpy)32+ and MV2+ were added to the clay colloid, which was stirred for a day. Then, an aqueous EDTA solution was added, 8563
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sample shows characteristic absorption bands at around 400 and 600 nm assigned to the MV+• species92 (Figure 1b) in addition to the band due to Ru(bpy)32+. The MV+• species does not form in the dark. These results demonstrate that photoexcitation of the Ru(bpy)32+ species with visible light induces the reduction of MV2+ to MV+•. On the other hand, the band due to Ru(bpy)32+ is retained during the photochemical reaction. This confirms the stability of Ru(bpy)32+. The occurrence of the PET reaction depends on the clay species and the %CEC value. Table 2 summarizes the results of
and the sample was stirred for a few minutes. We ensured that the total amount of the Ru(bpy)32+ and MV2+ did not exceed the CEC of the clay. We defined the percentage of the total amount of Ru(bpy)32+ and MV2+ relative to the CEC of clay as the %CEC value, which was set to 60 or 30%. The molar ratio of Ru(bpy)32+/MV2+/EDTA was usually set to 1/10/200. We express the samples with the clay species, %CEC value, and Ru(bpy)32+/MV2+/EDTA ratio like Hect 60 (1/10/200), where the clay is Hect, the %CEC value is 60, and the Ru(bpy)32+/MV2+/EDTA molar ratio is 1/10/200. Observation of the Photochemical Behavior. A suspension sample was placed in a water-cooled (298 K) quartz cell (5 mm in thickness) capped with a rubber septum. After bubbling with water-saturated nitrogen gas for more than 30 min, the sample was irradiated by a Ushio SX-UI500XQ 500 W Xe lamp for 10 min. Only visible light with wavelength longer than 440 nm was irradiated using a Toshiba Y-46 cutoff filter. Visible diffuse reflectance spectra of the sample were measured before and after the irradiation. During the experiment, the cell was stood with flowing nitrogen gas in the headspace and was kept static. Analyses. Visible spectra were measured by a Shimadzu UV-2450 spectrophotometer equipped with an integrating sphere. XRD patterns were recorded on a Rigaku Ultima IV diffractometer (monochromatic Cu Kα radiation). Fluorescence spectra were obtained by using a Jasco FP-6500 spectrofluorometer. Optical microscope observations were carried out with an Olympus BX51 microscope equipped with a BX2-FL-1 fluorescence unit.
Table 2. Summary of the PET Reaction in the Multicomponent Suspensions of Clay, Ru(bpy)32+, MV2+, and EDTA sample name Hect 60 (1/10/200) Hect 30 (1/10/200) Hect 60 (5/10/270) Hect 60 (1/10/20) Hect 60 (1/10/0) Hect 60 (1/10/200T)c Sapo 60 (1/10/200) Sapo 30 (1/10/200) FH 60 (1/10/200) FH 30 (1/10/200) FH 60 (5/10/270) Mont 60 (1/10/200) Mont 30 (1/10/200)
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RESULTS The PET Reaction in the Multicomponent Clay Suspensions and Effects of the Sample Composition. The PET reaction in the multicomponent system composed of the MV2+, Ru(bpy)32+, and EDTA molecules together with clay particles is typically observed in the Hect samples, where the MV2+ dications are reduced by irradiation of visible light with >440 nm to form MV+• radical cations that are detected spectroscopically. Figure 1 indicates visible diffuse reflectance spectra of the Hect 60 (1/10/200) sample before and after the visible light irradiation. The sample before the irradiation exhibits an absorption band at around 460 nm assigned to the Ru(bpy)32+ species1,91 (Figure 1a). After the irradiation, the
% CEC
molar ratioa
sample appearance
occurrence of PET
ΔR600b
60
1/10/200
flocculated
yes
0.026
30
1/10/200
flocculated
yes
0.013
60
5/10/270
flocculated
yes
0.035
60
1/10/20
flocculated
yes
0.008
60
1/10/0
flocculated
no
60
1/10/200
flocculated
yes
0.007
60
1/10/200
flocculated
yes
0.007
30
1/10/200
suspended
no
60
1/10/200
flocculated
no
30
1/10/200
suspended
no
60
5/10/270
flocculated
no
60
1/10/200
flocculated
no
30
1/10/200
suspended
no
a Ru(bpy)32+/MV2+/EDTA molar ratio. bDifference in the reflection intensity at 600 nm corrected with the Kubelka−Munk function before and after the irradiation. cTEOA is used as the sacrificial donor instead of EDTA
PET in the samples examined. The difference in the absorption intensity (Kubelka−Munk function) at 600 nm (ΔR600 value) before and after the irradiation is calculated as a rough measure of the amount of the photogenerated MV+• molecules. The photogeneration of MV+• species is observed for the Hect and Sapo samples but not for the FH and Mont samples when compared at the same %CEC value of 60. Hect and Sapo are characterized by small particle sizes (∼30 nm), whereas the particle sizes of FH and Mont are rather large (>0.1 μm). On the other hand, the Hect and Sapo samples show different behavior when the %CEC value decreases to 30. At this composition, the Hect sample shows the generation of MV+• ions, but the Sapo sample does not. Other components also affect the photochemical behavior. An increase in the amount of Ru(bpy)32+ relative to MV2+ with the constant %CEC value (i.e., the total amount of Ru(bpy)32+ and MV2+ with respect to CEC) yields the MV+• species with a greater amount as deduced from the comparison of Hect 60 (1/10/200) and Hect 60 (5/10/270). The FH 60 (5/10/270)
Figure 1. Visible diffuse reflectance spectra of the Hect 60 (1/10/200) sample (a) before the irradiation and after (b) 0, (c) 15, (d) 30, (e) 60, and (f) 90 min from the termination of the irradiation of 10 min. 8564
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Figure 2. Photographs of the multicomponent suspensions of clay, Ru(bpy)32+, MV2+, and EDTA species.
Figure 3. Photographs of the multicomponent suspensions after the PET reaction.
shown in Figure S1 of the Supporting Information, clearly indicates large background, which is much greater than that of a colloidal suspension of Hect without Ru(bpy)32+, MV2+, and EDTA. The large background with weak absorption because of Ru(bpy)32+ is ascribed to light scattering by the aggregated clay particles. The sample appearances also indicate that the photochemical reaction occurs in the solid phase. In our samples, almost all of the Ru(bpy)32+ and MV2+ species are adsorbed on the settled clay particles as obvious by the colorless supernatant shown in Figure 2 (also confirmed by spectroscopic analysis of the supernatant). This is ensured by the %CEC value being less than 100. The samples after the PET reaction all indicate the solid phase in greenish color as shown in Figure 3, which is rationalized by overlapping the colors of Ru(bpy)32+ (orange) and MV+• (blue). Lack of the color change in the liquid phase reveals that the PET reaction proceeds only between the Ru(bpy)32+ and MV2+ molecules adsorbed on the clay particles. The color change upon the irradiation is not observed for the inactive samples; photographs of some samples are shown in Figure S2 of the Supporting Information. Segregation of Ru(bpy)32+ and MV2+ Molecules on the Clay Particles. XRD measurements give evidence for the segregation of Ru(bpy)32+ and MV2+ on the clay particles. For the measurements, powder samples have been obtained by drying the suspensions under ambient conditions. Figure 4 shows XRD patterns of the dried powders obtained from the FH 60 (1/10/200), FH 60 (5/10/270), and Mont 60 (1/10/ 200) samples as the representatives that do not show the PET reaction together with those of the parent FH and Mont clays. The dried FH 60 (1/10/200) and Mont 60 (1/10/200) samples exhibit a diffraction peak at d ∼ 1.3 nm (2θ ∼ 6.7°) together with a weak shoulder at a lower 2θ angle. The shoulder becomes a distinct peak of d ∼ 1.8 nm (2θ ∼ 5°) when a larger amount of Ru(bpy)32+ is added as shown for the FH 60 (5/10/270) sample. This is also the case for the Mont 60 (5/10/270) sample as shown in Figure S3 of the Supporting
sample does not cause PET, and this result indicates that the clay species principally governs the reaction and that the sample composition is a subordinate factor. On the other hand, a decrease in the amount of EDTA yields the MV+• cations with a lower amount when the Hect 60 (1/10/200) and Hect 60 (1/ 10/20) samples are compared. The Hect 60 (1/10/0) sample lacking EDTA does not photogenerate the MV+• cations. The use of TEOA instead of ETDA as the sacrificial donor gives essentially the same result as deduced from the result of the Hect 60 (1/10/200T) sample where the letter T means the use of TEOA indicating that the anionic property of the EDTA does not crucially influence the PET reaction. Appearance of the Samples. The clay suspensions that show the PET reaction are not ordinary homogeneous suspensions93 but are the samples where the clay particles are flocculated and settled.94 Figure 2 shows digital camera images of the samples examined in the present study. All of them indicate orange color because of the Ru(bpy)32+ species. With the same Ru(bpy)32+/MV2+/EDTA ratio of 1/10/200, the Hect 30, Hect 60, Sapo 60, Mont 60, and FH 60 samples indicate flocculation of the clay particles while the clay particles are homogeneously dispersed in the Sapo 30, Mont 30, and FH 30 samples. The Hect 60 (5/10/270) and Hect 60 (1/10/200T) samples also show the flocculation of clay particles. This is rationalized by screening of the electrical double layers by the relatively high electrolyte concentrations of the samples with the %CEC value of 60; however, for Hect, the samples with the %CEC values of 60 and 30 both show the flocculation although the reason is unclear. These results demonstrate that the PET reaction in the Hect and Sapo systems is observed only when the clay particles are flocculated and settled whereas the FH and Mont systems do not show the photochemical response even if the clay particles are flocculated. Aggregation of the clay particles is also evidenced spectroscopically. We measured visible absorption spectra of the Hect 60 (1/10/200) sample with an ordinary transmittance method in order to confirm the aggregation. The spectrum, 8565
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Figure 5. Powder XRD patterns of (a) parent Hect, (b) dried Hect 60 (1/10/200) sample, (c) dried Hect 60 (5/10/270) sample, (d) parent Sapo, and (e) dried Sapo 60 (1/10/200) sample.
peak or small multiple peaks at around 2θ = 5−7°. This result means poor stacking regularity of the clay layers in these samples and lack of confinement of Ru(bpy)32+ and MV2+ molecules within the interlayer spaces; thus, the Ru(bpy)32+ and MV2+ molecules are not strictly separated by the clay layers. When the Ru(bpy)32+ or MV2+ species is solely added to the Hect colloid (the amount of Ru(bpy)32+ or MV2+ is set to 60% CEC), the dried sample indicates a clear basal diffraction peak corresponding to the intercalation compound of Hect with Ru(bpy)32+ or MV2+ as shown in Figure S4 of the Supporting Information. Assignment of each small bump observed in Figure 5 to bare or intercalated clay phases is unlikely because the XRD patterns of the Hect and Sapo solely intercalated with Ru(bpy)32+ or MV2+ (Figure S4 of the Supporting Information) exhibit the basal diffraction peaks much broader than the bumps. From these results, coexistence of the Ru(bpy)32+, MV2+, and EDTA species is indispensible for the poorly ordered aggregation of the clay layers. Spectroscopic Measurements. Photoluminescence spectra of the multicomponent clay suspensions provide other information of the Ru(bpy)32+ and MV2+ molecules adsorbed on the clay particles. Figure 6 shows the luminescence spectra of Ru(bpy)32+ and MV2+ species in the Hect 60 (1/10/200) and FH 60 (1/10/200) samples.The Hect 60 (1/10/200) sample shows intense luminescence of both Ru(bpy)32+ and MV2+ whereas FH 60 (1/10/200) gives much weaker luminescence of both the species. Weakened photoemission of luminophores, including Ru(bpy)32+ and MV2+, has frequently been reported when they are adsorbed on smectite-type clays.13,48,49,67,74,75,78,79,86 Clustering of the luminescent molecules induced by segregation in the interlayer
Figure 4. Powder XRD patterns of (a) parent FH, (b) dried FH 60 (1/10/200) sample, (c) dried FH 60 (5/10/270) sample, (d) parent Mont, and (e) dried Mont 60 (1/10/200) sample. Asterisks indicate the diffraction due to EDTA.
Information. Moreover, the FH 60 (10/10/360) and Mont 60 (10/10/360) samples, where the amount of Ru(bpy)32+ is larger than that in the previous samples, exhibit the 1.8 nm peak dominantly (Figure S3 of the Supporting Information). Thus, the relative intensity of the 1.8 nm and 1.3 nm peaks clearly depends on the relative amount of Ru(bpy)32+ and MV2+ added to the samples. The d values of 1.8 and 1.3 nm correspond to the basal spacings of the intercalation compounds of the smectite-type clays with the Ru(bpy)32+ and MV2+ species, respectively.75,77,95−97 The obtained XRD patterns are essentially the same as that reported by Villemure et al. for the segregative intercalation of the Ru(bpy)32+ and MV2+ cations into Mont.85 Hence, in the FH and Mont samples, the Ru(bpy)32+ and MV2+ molecules are segregated in the clay interlayer spaces to form their own intercalation compounds that are phase-separated from each other. Cointercalation is ruled out because the relative intensity of the basal diffraction peaks of Ru(bpy)32+ and MV2+ alters in accordance with the relative amount of Ru(bpy)32+ and MV2+. The Hect 60 (1/10/200), Hect 60 (5/10/270), and Sapo 60 (1/10/200) samples, which cause the PET reaction, give different XRD patterns. They do not clearly indicate the segregation of the Ru(bpy)32+ and MV2+ species. Their XRD patterns shown in Figure 5 exhibit a very weak broad diffraction 8566
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typical bright-field and fluorescence microscope images of the Hect 60 (1/10/200) and FH 60 (1/10/200) samples. Location of the Ru(bpy)32+ molecules is represented by their red emission. The bright-field image of the Hect sample indicates distribution of small particles, and the fluorescence image shows that each particle exhibits the red luminescence due to Ru(bpy)32+. Thus, macroscopic segregation (at the length scale of ∼10 μm) of the Ru(bpy)32+ molecules is unlikely. On the other hand, the bright-field image of the FH sample shows the presence of both orange-colored and almost colorless particles. The fluorescence image indicates inhomogeneous intensity of the emission; intense emission is observed in the area of orange-colored particles. These observations suggest the segregation of Ru(bpy)32+ molecules at a macroscopic scale on the FH particles. Although the inhomogeneous intensity can also be explained as uneven thickness of the clay particles, our interpretation based on the segregation is consistent with the results of XRD and spectroscopy as discussed later. PET Reaction in the System Added by Excess Ru(bpy)32+ and MV2+. We have prepared the FH 200 (1/ 10/200) sample, where the %CEC value is 200; in other words, the total amount of Ru(bpy)32+ and MV2+ greatly exceeds CEC of the clay. It contains Ru(bpy)32+ and MV2+ cations in the liquid phase. Figure 8 shows the photographs of the sample Figure 6. Photoluminescence spectra of (A) MV2+ and (B) Ru(bpy)32+ species in the (a) Hect 60 (1/10/200) and (b) FH (1/ 10/200) samples. Excitation wavelengths are 300 and 460 nm for the MV2+ and Ru(bpy)32+ species, respectively.
spaces has been recognized as a major origin of this behavior. Our result indicates that the luminescence intensity of the Ru(bpy)32+ and MV2+ species is greatly reduced in the FH 60 (1/10/200) sample where the luminophores are segregated as shown with the XRD measurement. This is in agreement with the previous studies of the luminescent species adsorbed on smectite-type clays with segregation. Optical microscopy allows evaluation of the distribution of the Ru(bpy)32+ molecules at micrometer level. Figure 7 shows
Figure 8. Photographs of the multicomponent suspensions of FH 200 (1/10/200), where the total amount of Ru(bpy)32+ and MV2+ is twice that of the CEC of clay before and after the visible light irradiation.
before and after the irradiation. The photograph indicates formation of the MV+• species in the liquid phase because the solution phase turns blue with the irradiation. This confirms the occurrence of the PET reaction84,85 in contrast with the other FH samples where the Ru(bpy)32+ and MV2+ cations are all adsorbed on the clay particles. Thus, the electron transfer between dissolved Ru(bpy)32+ and MV2+ (or either of them is adsorbed) is not suppressed by the clay particles, which is consistent with the studies of Villemure et al.84,85
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DISCUSSION The present study demonstrates that the PET reaction between the Ru(bpy)32+ and MV2+ species adsorbed on the smectitetype clay particles in water occurs in the presence of excess EDTA as the sacrificial donor under certain conditions although previous studies have reported suppressive effects of the clays on this reaction. The requisites for the occurrence of PET are the use of clays with small particle sizes (Hect and Sapo) and the flocculation of the clay particles where the Ru(bpy)32+ and MV2+ molecules are adsorbed. The isomorphous substitution position has little influence on the reaction because Hect (octahedral substitution) and Sapo (tetrahedral substitution) both enable the reaction. The particle size is a principal governing factor of the PET reaction since only Hect (Laponite) characterized by small particle size allows the
Figure 7. (a, c) Bright-field and (b, d) fluorescence optical microscope images of (a, b) Hect 60 (1/10/200) and (c, d) FH 60 (1/10/200) samples. 8567
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reaction between the two hectorite minerals examined (Hect and FH). Because Ru(bpy)32+ and MV2+ are both adsorbed on the clay platelets and thus their diffusion is greatly limited in our samples, the Ru(bpy)32+ and MV2+ molecules immobilized on the clay particles closely located to each other prior to the reaction can only contribute to the PET reaction. In other words, fast molecular diffusion of Ru(bpy)32+ and MV2+ is not important. Necessity of the flocculation of the clay particles for the PET reaction supports this idea because the Ru(bpy)32+ and MV2+ molecules can be placed with the close location under such conditions. Hence, the difference in the photochemical response between the Hect and Sapo samples and the FH and Mont samples, in all of which the clay particles are flocculated, is ascribed to different aggregated structure of the clay platelets. This supposition is also supported by the result of the FH 200 (1/10/200) sample that contains excess amounts of Ru(bpy)32+ and MV2+. This sample undergoes the PET reaction with the contribution of dissolved Ru(bpy)32+ and MV2+ ions. Thus, we suppose that the PET reaction in the FH and Mont samples is suppressed only when Ru(bpy)32+ and MV2+ are both adsorbed on FH and Mont. Nevertheless, the Hect and Sapo samples allow the reaction even when Ru(bpy)32+ and MV2+ are both adsorbed. Hence, we conclude that the key to the reaction is the aggregated or assembled state of the clay particles. The XRD measurements indicate that the difference in the particle size is related to the aggregated state of the clay layers and the segregation of Ru(bpy)32+ and MV2+ species. The diffractograms of the clay minerals with large particle sizes (FH and Mont) show clear evidence for the segregation by which the Ru(bpy)32+ and MV2+ molecules are incorporated into different interlayer spaces, which is consistent with a previous study.85 Moreover, the optical microscope observations of the FH sample implies that the location of the Ru(bpy)32+ molecules is inhomogeneous at macroscopic length scale (∼10 μm). In this state, the spatial separation between the Ru(bpy)32+ and MV2+ molecules should be large enough to hinder the PET reaction. Formation of the two distinct intercalation compounds with Ru(bpy)32+ and MV2+ indicates their separation at a certain distance; close location sandwiching a single clay layer illustrated in another previous study67 is unlikely. The XRD patterns also indicate high stacking regularity of the clay layers so that the Ru(bpy)32+ and MV2+ molecules are firmly confined in the interlayer spaces. On the other hand, the clay minerals with small particle sizes (Hect and Sapo) show great broadening of the basal reflection peak in the XRD patterns. This demonstrates poorly ordered aggregation of the clay layers as typically exemplified by houseof-cards type aggregation.51 Thus, the Ru(bpy)32+ and MV2+ molecules are not confined in the interlayer spaces but are exposed to external species. This situation allows close contact of the Ru(bpy)32+ and MV2+ molecules to enable the PET reaction. The optical microscope observations do not evidence macroscopic separation of the Ru(bpy)32+ and MV2+ molecules. However, we suppose that the segregation of Ru(bpy)32+ and MV2+ is retained even in the Hect and Sapo samples. Our results clarify that the PET reaction occurs only when the clay particles adsorbing the Ru(bpy)32+ and MV2+ molecules are flocculated and settled. Although the Hect 30 (1/10/200) sample where the flocculation is observed causes the reaction, the Sapo 30 (1/10/200) sample in which the clay particles are well dispersed does not. Thus, the Ru(bpy)32+ and MV2+
molecules meet only when the clay particles are aggregated. This supports the segregative adsorption of Ru(bpy)32+ and MV2+. In the flocculated state, the adsorbed Ru(bpy)32+ and MV2+ molecules which are exposed to external molecules because of the poor stacking of the clay layers can contact to transfer electrons. The photoluminescence spectra can be explained along this line. Although the Hect 60 (1/10/200) sample exhibits definite luminescence of the Ru(bpy)32+ and MV2+, the FH 60 (1/10/ 200) sample shows weak emission of both of the species. Both of the clay minerals are synthetic samples and do not contain iron species that can work as a quencher for both of the luminophores. Thus, the difference in the luminescence intensity of the Hect and FH systems is ascribed to the microenvironments provided by the clay layers. For the FH sample, the segregated Ru(bpy)32+ and MV2+ molecules will be ordered in the interlayer spaces to form the well-organized intercalation compounds as shown by XRD. This situation would cause rather dense arrangement of the molecules in the interlayer spaces to induce self-quenching of the luminescence. In the Hect sample, however, the microenvironments of the Ru(bpy)32+ and MV2+ molecules would not be homogeneous because of the disordered structure. In this state, the luminophore molecules can avoid self-quenching to a certain extent depending on their adsorbed sites to exhibit strong emission. Although other possibilities such as different intrinsic properties of Hect and FH are not ruled out at present as the interpretation of the weak emission of the FH sample, clustering of the luminophores explains the spectroscopic and other analytical data without conflict. These considerations lead to a conclusion that the suppressive effect of the clays on the PET reaction between the Ru(bpy)32+, MV2+, and EDTA molecules is avoided if the multiscale structure of the system is properly organized. The higher-order structure constructed by the aggregation of the clay particles principally governs the PET reaction even if molecular-level interactions, only which have been taken into account in many previous studies, between the clay particles and the adsorbed species are resembled. In the present study, the appropriate higher-order structure is obtained by the use of the clay minerals with small particle sizes. Among previous studies, Tajik and Detellier have compared photoinduced hydrogen evolution in suspensions of natural hectorite and montmorillonite where a cationic rhenium bipyridine complex adsorbed on the clay particles is used as a photocatalyst and have found that only the hectorite suspension generates hydrogen.98 Although detailed characterizations have not been conducted, their result is in accordance with the present study. The negative effect of smectite-type clays as the matrix of PET reaction between Ru(bpy)32+ and MV2+ should be recognized as an oversimplified rule.
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CONCLUSIONS In summary, the present study demonstrates that multicomponent systems composed of Ru(bpy)32+, MV2+, EDTA, and smectite-type clay particles are utilizable for the PET reaction if the sample conditions are appropriately adjusted although the clays have been believed to inhibit the reaction for a long time. The reaction is initiated by photoexcitation of the Ru(bpy)32+ cations with visible light and is followed by electron transfer to the MV2+ cations and regeneration of the Ru(bpy)32+ species through electron donation from the sacrificial EDTA anions. The clay particles gather the 8568
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Ru(bpy)32+ and MV2+ molecules into the heterogeneous microenvironments to facilitate the PET reaction. Segregative adsorption of Ru(bpy)32+ and MV2+ on the clay particles is overcome through less ordered stacking and flocculation of the small clay particles by which the Ru(bpy)32+ and MV2+ molecules are exposed to each other to transfer electrons. The inhibition of the reaction in the samples where the clay particles are well dispersed indicates a critical role of the higherorder structure constructed by the aggregated clay particles. Such hierarchical structures of the clay particles will be utilized for constructing other types of photoactive integrated systems based on clay minerals.
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ASSOCIATED CONTENT
S Supporting Information *
Visible transmission spectra of a suspension sample and a hectorite colloid, photographs of some photoinactive multicomponent suspensions, XRD patterns of some samples. This information is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone, fax: +81-93-884-3308. E-mail:
[email protected]. jp. Present Address †
Graduate School of Engineering, Kyushu Institute of Technology, 1−1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka 804−8550, Japan.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciate Dr. Shun-ichi Ohta (Topy Industries Ltd.) for the kind gift of the fluorohectorite sol. We also thank Ms. Takako Fujita (Tokyo University of Agriculture and Technology) for her assistance with experiments. This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas of Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control (Area No. 2206, Grant No. 23107511) from the Ministry of Education, Culture, Sports, Science, and Technology.
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